This invention relates to an aluminum base powder alloy and metal articles made therefrom and a method of making metal articles. More particularly, it relates to an aluminum base powder alloy, containing silicon and manganese, produced at a relatively low alloy atomizing temperature and to a method of producing metal articles such that an article made therefrom has a relatively low coefficient of thermal expansion and high strength at elevated temperatures.
Aluminum and aluminum base alloys have certain characteristic advantages over other metals and metal alloys; one such advantage is their light weight. With the weight of materials becoming increasingly important, as for example, the concern to decrease the weight of motor vehicles, such concern has resulted in the increasing use of aluminum. Aluminum base alloys can be suitable for use in pistons for internal combustion engines and with such use are subjected to severe working conditions.
The pistons and the engine block may be subjected to different thermal conditions, and thus different expansion rates, since the engine block temperature may be lower than the piston temperature as a result of a coolant circulating through the engine block. Such is the case whether or not the piston and block are made of the same metal alloys, as steel, cast iron, or aluminum. The problem is accentuated, however, when the piston and block are of dissimilar metals. The piston which may be the hottest portion of the engine should have thermal expansion properties which will enable it to maintain its dimensional stability relative to the engine block over a temperature range higher than the engine block. The strength of the material should also be maintained over such higher temperatures. It is especially desirable, therefore, that such aluminum base alloys have a relatively low coefficient of thermal expansion and be able to maintain a relatively high strength at elevated temperatures for an extended period of time.
Aluminum base alloys containing relatively large amounts of silicon and manganese have been used in cast articles. U.S. Pat. No. 1,829,668, issued Oct. 27, 1931, discloses a cast aluminum base alloy containing 4 to 13% silicon and 4 to 13% manganese. Cast pistons made from aluminum base alloys containing silicon and manganese are also disclosed in the prior art. U.S. Pat. No. 2,185,348, issued Jan. 2, 1940, relates to an aluminum base alloy containing up to 13% silicon, up to 3% manganese, and significant amounts of iron, antimony and a metal from a tungsten group. U.S. Pat. No. 2,357,451, issued Sept. 5, 1944, discloses an aluminum base alloy having 18 to 35% silicon, up to 1% manganese, up to 1% magnesium, and significant amounts of copper, iron, tin and zinc.
It is also recognized in the art to use powder metallurgy (P/M) techniques in high strength dispersionhardened alloys with elements that normally cannot be cast in aluminum without difficulty. Aluminum base alloy powder containing significant amounts of magnesium, silicon and manganese is disclosed in U.S. Pat. No. 2,287,251, issued June 23, 1942. Using a powder form of aluminum base alloys containing significant amounts of silicon for making pistons is disclosed in U.S. Pat. No. 2,978,798, issued Apr. 11, 1961 and U.S. Pat. No. 3,282,745, issued Nov. 1, 1966. Such alloys for making powder should have a low liquidus temperature, i.e. low melting point of the alloy, to simplify the production of powder by using less expensive and less complex equipment. Typically, a suitable molten alloy can be atomized at a temperature higher than the liquidus temperature of the alloy. Such a higher temperature is used to ensure successful atomization with many aluminum alloys being atomized in excess of 1625.degree. F. (1158.degree. K.). In practice, there is some cooling of the molten metal during atomization due to an atomizing gas temperature which is usually lower than the liquidus temperature. When that is the case, then a metal atomizing temperature greater than the liquidus temperature is used. An atomizing temperature of about 1650.degree. F. (1172.degree. K.) or more, however, becomes impractical from a cost standpoint.
The aluminum base powders of the prior art do not provide the characteristics and properties desired in metal articles that undergo the stresses and temperatures such as may be prevalent with automotive pistons and the like. For instance, it is desirable to provide an alloy which can take the advantages of powder metallurgy techniques and can offer relatively high strength at temperatures above 400.degree. F. (478.degree. K.) and a low thermal expansion coefficient of less than 11.0 .times. 10.sup.-6 inch/inch/.degree. F. over a range of 0 to 250.degree. F. (255 to 394.degree. K.) and can be melted and atomized at a temperature as low as about 1400.degree. F. (1033.degree. K.) to a relatively fine powder.